This invention relates to in situ production of gas from an underground formation of carbonaceous material and in particular to a process in which gas production is achieved by applying a direct electric current to the formation.
The production of gaseous and liquid hydrocarbons by in situ gasification of underground formations of carbonaceous substances, such as coal, oil shale, and the like has long been recognized as a means of avoiding the high costs and inefficiencies attendant fuel production by conventional methods which rely on underground mining operations to provide feed stocks.
Among the prior art methods which have been proposed for in situ gas production are those involving combustion of the carbonaceous material in the subterranean formation. In one such method, a combustion zone is established by depositing combustible material in fractures in the formation adjacent to a well-bore, and passing sufficient current between electrodes positioned in well-bores connected with the fractures so as to heat the combustible material to its ignition temperature. Combustion is supported by the injection of oxygen or air through the well-bore into the combustion zone. As the injection of the combination supporting medium continues, the combustion front is driven radially outwardly from the injection well along the fractures. Gaseous hydrocarbons driven out of the formation by the combustion process are recovered from a production well penetrating the formation. See, for example, Dixon, U.S. Pat. No. 2,818,118. Related combustion processes involving electrocarbonization of underground formations to achieve in situ gas production are disclosed in Sarapuu, U.S. Pat. No. 2,795,279 and Parker, U.S. Pat. No. 3,106,244.
Other proposed in situ gasification methods have involved the use of electrical energy to heat the formation directly. For example, Baker, U.S. Pat. No. 849,524, describes a method in which electric current is passed through an underground formation by means of conductors placed in well-bores penetrating the formation, thereby heating the formation and volatizing components thereof, which are recovered through one of the wells. Although the Baker patent does not give the conditions employed in carrying out the method, temperatures in excess of 650.degree. F. are generally necessary to produce fuel gas by pyrolysis of oil shale, tars and coal.
A related method specific to the treatment of oil shale formations is disclosed in Parker, U.S. Pat. No. 3,428,125. The method entails injecting an electrolyte into the formation through two or more well-bores and applying an electrical potential across the formation between the well-bores. An electric current passes through and heats the formation to a temperature sufficient to pyrolyze the hydrocarbons present in the oil shale, while back-pressure is maintained on the formation to prevent vaporization of the electrolyte.
Although the prior art methods referred to above demonstrate that electrical energy can be used successfully for the in situ production of fuel gas, those methods have some rather serious shortcomings.
Combustion processes produce gas which is diluted with combustion products, as well as nitrogen gas in those instances where air is employed to sustain combustion. Dilution occurs as a result of channeling or formation collapse which allows the diluents to break-through the combustion front and become intermixed with the gases preceding it. These are natural consequences of combustion processes about which nothing can be done. Hence, while a relatively high Btu content gas is swept in front of the expanding combustion front, the effects of channeling and formation collapse are such that the average Btu value of the gas actually recovered by combustion processes is relatively low, ranging anywhere from 100-1000 Btu/cu.ft. and usually toward the low end of this range.
Electrical methods such as those described in Baker, U.S. Pat. No. 849,524 and Parker, U.S. Pat. No. 3,428,125 require that a temperature on the order of 500.degree. F. to 660.degree. F. be maintained in the underground formation for successful operation. The amount of energy required for heating the formation to within this range is substantial. As stated in the Parker patent, for example, an electrical potential in excess of 400 volts must be impressed across the well casings with sufficient back-pressure of up to 1530 psig. applied on the well-bores to maintain the required temperature in the formation. In view of the ever-increasing costs of electrical energy, the operating conditions of these prior art methods must be considered a severe drawback.
A recent article by Coughlin et al, Nature, Vol. 279, pp 301-03 (1979) reports on an improved electrical method for coal gasification. In this method, a coal slurry undergoes treatment in an electrochemical cell, which is divided into separate anode and cathode compartments, to produce essentially pure hydrogen at the cathode, and CO.sub.2, containing small amounts of CO (about 3% at steady-state) at the anode. The method is carried out at relatively moderate temperatures and electrical potentials. For example, lignite reportedly has been gasified at potentials from 0.85 to 1.0 volts at about 240.degree. F. While this method has been practiced on a laboratory scale, its commercial practicability has yet to be demonstrated. Moreover, even if it is operative on a commercial scale, the operating cost thereof would be relatively high, since it would require mined coal for the feed stock. Further, the mixture of gases produced by this method has a lower Btu value than is acceptable for a fuel gas.
The desirability of a commercially practical method for producing a high Btu fuel gas by the use of electrical energy under relatively moderate operating conditions in areas where existing recovery technology has not been effective has lead to the development of the present invention.